Perforated transparent glazing for heat recovery and solar air heating

ABSTRACT

A heat collector comprises a transparent glazing exposed to the ambient. The transparent glazing is spaced from a back surface to define a plenum therewith. A plurality of perforations is defined through the transparent glazing for allowing outside air to flow through the transparent glazing into the plenum and substantially maintain the transparent glazing at the ambient temperature, thereby providing for higher thermal efficiency.

RELATED APPLICATION

The present application is a continuation-in-part of U.S. patent application Ser. No. 12/178,211 filed on Jul. 23, 2008 the content of which is incorporated herein by reference.

TECHNICAL FIELD

The present application generally relates to a device suited for pre-heating fresh outside air by means of free energy, such as solar energy and/or heat recovery.

BACKGROUND ART

Design of traditional glazed solar air heaters generally comprises a glass, polycarbonate or Lexan® transparent cover placed in front of a dark solar absorber. The front transparent cover is provided for minimizing heat losses from the top of the collector. Fresh outside air is traditionally admitted at one end of the collector between the front transparent cover and the solar absorber. The air passes through the collector along fins and absorbs heat from the solar absorber as it travels therealong. Warm or hot air is discharged at the opposite extremity of the collector. As air progresses inside the collector, its temperature rises above ambient. The higher the temperature in the collector is, the higher the heat loss towards the ambient becomes. Heat loss happens through the bottom, the edges and the top (where the glazing is) of the collector. Typically the edges and the bottom are insulated, so that heat loss mostly occurs through the top, that is by convection between the absorber and the glazing and then by conduction through the glazing. When the glazing becomes very warm, the collectors become less efficient.

Various unglazed solar air heaters have also been designed over the years. Current transpired collector designs are such that the solar absorbing surface is located outside facing the sun, unprotected by means of a glazing. The perforated absorber is coupled to a fan which creates a negative pressure between the building (or the bottom of the collector) and the absorber. When the fan is in operation, the air is drawn through the absorber. The air passing through the perforations in the outer opaque absorber breaks the naturally occurring warm film of air on the outside facing side (the boundary layer) of the absorber. This method provides acceptable performances when the flow of air per unit area exceeds 6 cfm per square foot of collector. However, for unitary flow rates below 5 cfm per square foot, the amount of cool air leaching the perforated plate is insufficient to prevent the collector plate from heating up, thereby negatively affecting the overall thermal efficiency of the system. Efficiencies at the rate of 2 cfm per square foot drop to 30% or even less.

SUMMARY

It is therefore an aim to address the above mentioned issues.

Therefore, in accordance with a general aspect of the present application, there is provided a method of improving the efficiency of a glazed solar collector comprising a glazed cover, a solar absorber disposed behind the glazed cover, and a plenum between the glazed cover and the solar absorber, the glazed cover forming an outer surface of the collector; the method comprising: providing multiple perforations through the glazed cover; and reducing heat looses to the environment through the glazed cover by minimizing a temperature delta across the glazed cover, including cooling the glazed cover by drawing outside air through the multiple perforations at a flow rate between about 2 to about 6 cfm per square foot of glazed surface.

In accordance with another general aspect, there is provided a glazed solar air collector comprising a perforated glazed cover transparent to solar radiation, the perforated glazed cover having opposed front and back faces, the front face of the perforated glazed cover forming an external surface of the collector and being directly exposed to the ambient, a solar radiation absorbing panel disposed behind the perforated glazed cover for absorbing solar radiation passing through the perforated glazed cover; a plenum defined between the back face of the perforated glazed cover and an opposed front face of the solar radiation absorbing panel, the perforated glazed cover having a plurality of perforations distributed over a surface area thereof and collectively forming a main outdoor air intake for admitting fresh outdoor air into the plenum, the distribution of perforations being selected to maintain a temperature delta across the perforated glazed cover close to zero, a secondary outdoor air intake provided at least one of a bottom and a side of the collector and disposed to direct an additional flow of outdoor air over predetermined surface areas of the solar radiation absorbing panel prone to overheating, and air moving means to draw heated air from said plenum via an outlet thereof.

In accordance with still another general aspect, there is provided a transparent and perforated surface exposed to the ambient. The perforated transparent surface is spaced from a back surface so as to define an air gap or plenum therebetween. Fresh outside air is drawn into the plenum through the perforated transparent surface. The back surface can, for instance, be provided in the form of a bottom of a solar collector, a building wall or roof, an outer surface of a greenhouse, a photovoltaic panel, the ground or any non-porous surface. Between the perforated transparent surface and the back surface, the gap of air is maintained under negative pressure due to mechanical or natural means. An outlet is provided for allowing the air flowing through the plenum to be drawn into a duct or a channel, for use as make-up, ventilation, process or combustion air to a device which consumes or needs thermal energy.

The air in the plenum is heated either by incident solar radiation on the surface of the back panel, which acts as a solar absorber, and/or by heat escaping from the back surface. The device can therefore act as a solar air heater and/or as a heat recovery unit. When used as a solar air heater, the back surface can be of a dark color, so that incident solar radiation passing through the perforated transparent surface is absorbed by the back surface in the form of heat and not reflected back to outer space. However, if the back surface, for any aesthetic reason or other, must be of light color, the solar thermal efficiency remains higher than other conventional unglazed collector design. This is particularly true when the device is used as a heat recovery device, since the back surface can be of any color with no influence on efficiency (it can even be transparent like in the case of a greenhouse), but the lower the thermal resistance (insulation) of the back surface, the greater the heat recovery rate. The device can be simultaneously used for both functions of solar heating and heat recovery.

If necessary, the preheated air leaving the device can have an auxiliary heating device located downstream (e.g. a gas-fired system) to bring its temperature to a given set point.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic side view of a solar collector including a perforated transparent cover in accordance with an embodiment of the present invention;

FIG. 2 is a schematic side view of another embodiment of a solar collector having a perforated transparent glazing;

FIGS. 3 and 4 are schematic side views of ground-mount configurations of solar collectors having perforated transparent glazing in accordance with further embodiments of the present invention;

FIG. 5 is a schematic side view of a wall mounted solar collector having a perforated transparent glazing;

FIG. 6 is a schematic side view of a roof mounted solar collector having a perforated transparent glazing;

FIG. 7 is a schematic view illustrating a perforated transparent glazing surrounding a greenhouse shell for pre-heating cold outside air before being drawn into the greenhouse by a ventilation system;

FIG. 8 is a graphic comparing the efficiency of perforated glazing collectors vs. unglazed perforated collectors as a function of the quantity of air flowing therethrough.

FIG. 9 a is a schematic side view of a perforated glazed solar collector adapted to be installed on an outer wall of a building for heating fresh outside air, the collector having a main outdoor air intake through the front perforated glazed cover and a secondary outdoor air intakes in the bottom of the collector and at the sides thereof for preventing hot air stagnation in the collector and cooling the solar absorber back panel;

FIG. 9 b is a schematic front view of the solar absorber back panel of the collector shown in FIG. 9 a and illustrating the secondary outdoor air intakes used to ensure a balance flow of air over the entire surface of the back panel, thereby avoiding the formation of hot spots thereon;

FIG. 10 is a schematic side view of a wall-mounted perforated glazed solar collector and illustrating the tapering profile of the plenum between the front perforated glazed cover and the solar absorber back panel, the width variation of the plenum ensuring minimum linear velocity of air over the surface of the absorber panel;

FIG. 11 is a front view of a perforated glazed panel that can be assembled in a co-planar relationship with other similar panels to form the outer perforated glazed cover of a solar collector;

FIG. 12 a is a cross-section view taken along line 12 a-12 a in FIG. 11;

FIG. 12 b is an enlarged fragmentary view of a top edge detail of the panel shown in FIG. 11;

FIG. 12 c is an enlarged fragmentary view of a bottom edge detail of the panel shown in FIG. 11;

FIG. 13 is an enlarged fragmentary view of the panel shown in FIG. 11 and illustrating vertical and horizontal thermal expansion clips integrally formed in the top and side edges of the panel;

FIG. 14 is an enlarged fragmentary view of one air inlet hole detail of the panel shown in FIG. 11 and illustrating a hole peripheral protuberance which projects outwardly from the outer surface of the panel around each of the perforations defined therethrough, the protuberances promoting turbulences as outside air is drawn through the perforated glazed panel;

FIG. 15 is an enlarged view of the outer surface panel detail encircled on FIG. 11 and illustrating how rain water flows at the outer surface of a vertically installed perforated glazed panel; and

FIG. 16 is a cross-section view taken along line 16-16 on FIG. 15 and illustrating water drops circulating around the holes on the panel.

The term “glazing” is herein intended to broadly refer to any transparent surface allowing the light to pass therethrough.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1 to 7 illustrate various embodiments of glazed outdoor air heating solar collectors.

More particularly, FIG. 1 shows a solar air heater 10 provided in the form of an elongated conduit-like enclosure mounted on a base and including a sun facing perforated transparent glazed cover 12 exposed to the ambient and placed in front of a back panel having an arcuate solar radiation absorber plate 14 applied over an insulation layer 15. The back panel is generally provided in the form of a half-pipe wall covered with the perforated transparent glazing 12. The absorber plate 14 can be of a dark color to maximize solar gain. The perforated glazing 12 can be provided in the form of a perforated polycarbonate or transparent UV-resistant plate. Other suitable sun ray transmissive polymers could be used as well. The glazing 12 can be rigid or flexible. The perforations in the glazed cover can be distributed over the entire surface of the glazing or over only a selected surface area thereof. The density of perforations can be uniform or variable over the glazing surface. As will be seen hereinafter, in addition of providing an air intake for the solar air heater 10, the perforations are configured to perform a cooling function in order to maintain the glazed cover at the ambient temperature.

The perforated glazing 12 and the solar radiation absorber plate 14 define a plenum 16 therebetween. A fan or other suitable air moving means 17 is operatively connected to an outlet 18 provided at one end of the back panel to draw fresh outside air through the perforated glazing 12 into the plenum 16 before being directed to a ventilation system, such as a building ventilation system. All the air admitted or fed into the plenum 16 is fresh outdoor air drawn from the environment. As can be appreciated from FIG. 1, the width of the air gap or plenum 16 gradually increases towards the outlet 18. Such a configuration may be used to avoid efficiency looses due to hot air stagnation in the plenum and heat radiation from the solar absorber plate 14. By gradually increasing the width of the plenum 16 towards the outlet 18, a sufficient linear air flow of outside air may be provided over the entire surface the solar absorber 14 to prevent the formation of hot spots, thereby minimizing radiation losses due to an overheated absorber. The entire surface of the solar absorber 14 may remain cooler and radiation therefrom may be reduced. Providing a more uniform temperature distribution over the entire surface of the absorber 14 thus contributes to improve the efficiency of the solar air heater 10.

The solar rays passing through the glazed cover, i.e. the perforated transparent glazing 12, are absorbed by the absorber plate 14. The air in the plenum 16 picks up the heat absorbed by the solar absorber before being drawn out of the plenum 16. As air travels longitudinally along the plenum 16 between the absorber plate 14 and the perforated glazing 12, additional fresh outside air is drawn through the perforated glazing 12. The perforated glazing 12 traps the heat within the plenum 16 until the heated air is drawn out of the heater via outlet 18. The influx of fresh outdoor air through the perforated transparent glazing 12 cools down the glazing 12 continuously, thereby preventing same from warming up. In this way, the glazing 12 remains at a temperature substantially equal to the ambient temperature. Accordingly, the temperature differential between the incoming air and the ambient is equal to zero or close to zero, so that thermal efficiency remains at the highest possible value. Heat losses which would otherwise occur with conventional uncooled glazed covers can thus be reduced to a minimum. The perforations in the glazed cover provide a simple and efficient cover cooling means. Integrating the cooling and air intake function in the glazed cover allows improving the efficiency of glazed ambient air heating solar collectors. Cooling the glazed cover by controlling parameters such as holes size, hole shape and distance between holes, as well as the geometry and the shape of the plenum allow to maximize heat recovery. For the heat to be removed over the surface of the perforated cover, the incoming air must efficiently “sweep” over the entire outer surface of the cover. In some applications, it is advantageous that the perforations in the glazed cover be as small as possible, i.e. the glazed cover should be as porous as possible. However, the diameter of the perforations may be limited by the manufacturing process of the glazed cover. For instance, for an injected molded glazed cover, it might be challenging to form the glazed cover with perforations having a diameter smaller than the 2 mm (0.08 inches). For 2 mm (0.08 inches) hole diameter and a nominal unitary flow rate of 4 cfm/sq.ft., or 75 m³/h per m² of collector, for which optimum performance is wished for, applicant measured with thermography and small scale an effective “heat removal radius” of 1 cm (0.4 inches) around each hole, when side wind velocities are below 3 m/s. The hole spacing shall therefore be dimensioned so as to allow at least 100% of the collector surface covered by the heat removal surface.

heat area hole hole collector removal covered exposed dia. spacing porosity radius area % to wind mm mm % mm % % 2 6 8.7 10 873% 0% 2 8 4.9 10 491% 0% 2 10 3.1 10 314% 0% 2 12 2.2 10 218% 0% 2 14 1.6 10 160% 0% 2 16 1.2 10 123% 0% 2 20 0.8 10  79% 21% 2 24 0.5 10  55% 45% 2 28 0.4 10  40% 60% 2 32 0.3 10  31% 69% 2 36 0.2 10  24% 76% 2 40 0.2 10  20% 80%

From the table above, it can be seen that for an embodiment having 2 mm hole diameter perforations, a hole spacing of a maximum of 16 mm should be used to allow over 100% the collector surface to be covered by the heat removal surface and be incentive to winds below 3 m/s (3 m/s is the side wind velocity used to rate air collectors by the SRCC).

FIG. 2 shows a second embodiment in which like reference characters refer to like components. The solar air heater 10 a shown in FIG. 2 essentially differs from the solar air heater 10 shown in FIG. 1 in that the solar air heater 10 a has a planar configuration characterized by spaced-apart parallel transparent glazing and back panel. The back panel is provided in the form of a flat absorber plate 14 a applied over a planar layer of insulation material 15 a. The absorber plate 14 a could be corrugated. Sidewalls or supports 19 a are provided along the perimeter of the back panel and the perforated transparent glazing 12 a in order to create a uniform air gap 16 a therebetween. The perforated glazing 12 a and the back panel are preferably co-extensive. The back panel 14 a can be provided in the form of photovoltaic (PV) panels to provide the double function of air heating and cooling the PV panels, which produce more electricity when their surface is kept at cool temperatures. As shown in FIGS. 1 and 2, the perforated transparent glazing 12 a is preferably supported at an inclination equal to the latitude of a given location, and facing the equator, depending on use. However, it is understood that the transparent glazing could be oriented and inclined otherwise. For instance, FIG. 4 shows a horizontally oriented perforated transparent glazing, whereas FIG. 5 shows a vertically oriented glazing.

As shown in FIGS. 3 and 4, the solar air heater can be mounted directly on the ground, the ground surface forming the back panel of the device. In the embodiment of FIG. 3, wherein like reference characters refer to like components, the plenum 16 b is formed by the perforated transparent glazing 12 b, a building wall 20 b and the ground G. The fresh outside air drawn in the plenum 16 b is heated by the solar radiations absorbed by the ground G as well as by the heat escaping from the building through wall 20 b. The heat escaping from the building wall is depicted by arrow A. The solar air heater is only fed with outside air through perforations defined in the perforated glazing 12 b. However, as will be seen herein after, secondary outdoor air intakes could be provided at the bottom or at the sides of the heater to provide a sufficient flow of air over the solar absorber backing and thus prevent that some areas thereof be overheated. The fresh outside air flowing through the perforations defined in the transparent glazing 12 b maintains the temperature delta across the glazing close to zero, thereby ensuring high thermal efficiency. The heated air is drawn out from the plenum 16 b and circulated in the building B via the building ventilation system (not shown).

As shown in FIG. 3, the plenum 16 b may have a flaring profile gradually widening towards the outlet end of the plenum 16 b. Applicant has found that by drawing fresh outside air through the perforated glazing 12 b and by increasing the cross-section of the plenum toward the outlet end thereof, hot air stagnation over the sun ray absorbing surfaces (i.e. the building wall and the ground in this illustrated example) and radiation losses may be reduced.

As shown in FIG. 4, where like reference characters again refer to like components, the solar air heater can also be provided in the form of an enclosure having a perimeter wall 19 c, a closed bottom end formed by the ground, and a top end covered by the perforated transparent glazing 12 c. An outlet 18 c connected to suitable air moving means is provided for withdrawing the heated air from the enclosure.

As shown in FIGS. 5 and 6, the perforated transparent glazing 12 d and 12 e can be mounted in opposed facing relationship to a building wall 20 d or the roof 22 e of a building to form a single-pane solar air heater (i.e. the only layer of material which needs to be installed over the building outer surface is the perforated glazing. This allows for a simple and cheap solar air heater installation. In the embodiment of FIG. 5, the plenum 16 d is formed between the outside surface of the building wall 20 d and the adjacent vertically oriented perforated transparent glazing 12 d. In the embodiment of FIG. 6, the plenum 16 e is formed by the outside surface of the building roof 22 e and the perforated transparent glazing 12 e. As depicted by arrows A, the heat escaping from the building envelope through the wall 20 d or the roof 22 e may be recovered to heat the air in the plenum 16 d and 16 e. The roof 22 e and the building wall 20 d both act as solar radiation absorbers to further heat the ambient air drawn in the plenums 16 d and 16 e through the perforated glazing 12 d, 12 e. The plenums 16 d and 16 e are only fed with fresh outside air and that irrespectively of the outside temperature. For instance, during winter time, outside air is still admitted into the plenums 16 d and 16 e. The solar radiations pass through the perforated transparent glazing and are absorbed by the underlying building wall or roof surfaces and the air in the plenum absorbs the heat from the building wall or roof. As opposed to conventional solar walls or solar roofs wherein solar radiation are directly absorbed by dark panels covering the wall or roof of the buildings, the transparent glazing does not negatively alter the appearance (i.e. change the color of the building wall or roof) of the building. Unlike the prior art, the performance of the system is not influence or restricted by the color of the perforated panels installed on the building wall or roof. The perforated glazing 12 d and 12 e are transparent and, thus, they do not change the color of the building wall or roof. No compromise has to be done for aesthetic purposes.

FIG. 7 shows a further potential application of the present invention. More particularly, FIG. 7 illustrates a greenhouse B′ having a skeleton framework covered with a transparent skin 25 f or membrane, as well know in the art. A perforated transparent glazing 12 f is mounted to the greenhouse wall and roof to define a double-walled structure including an air gap 16 f defined between the perforated transparent glazing 12 f and the inner transparent skin 25. In this embodiment, the perforated transparent glazing 12 f acts as a second insulation layer for the greenhouse B′. As depicted by arrows A, the heat escaping from the greenhouse through the inner skin 25 f is recovered in the air gap 16 f. The air admitted in the plenum 16 f is only outside air. A fan or the like can be provided for drawing heated air from the air gap back into the greenhouse B′. The perforated transparent glazing 12 f maintains the required transparency required for plant growth.

As can be appreciated from the above embodiments, the device can be used in several applications including:

Solar thermal air heaters

Solar fresh air preheater mounted on building walls or roofs

Hybrid solar air/water heating systems

Preheating of air-to-air and air-to water heat pumps

Transparent energy recovery device for greenhouses

Cooling of photovoltaic panels

Residential, low-cost solar preheater

Also various apparatus can be provided downstream of the device for further processing the air. For instance, the device could be coupled to the following units:

Gas-fired make-up air unit

Air-based heat pump (air-to-air or air-to-water)

Swimming pool heat pump

Combustion chamber

Heat recovery unit

The above described transpired or perforated glazing offers numerous benefits. The incoming air is admitted throughout the glazing surface, either on a large proportion of its surface or over the entire surface. Accordingly, the glazing surface remains cold so that collector top heat loss is substantially prevented. Furthermore, the air temperature inside the collector remains relatively cold, lowering heat losses through the bottom and the edges. The proposed perforated transparent glazing design provides solar efficiencies at least as good as that provided by the perforated plate design at high flow rates. For lower flow rates, however, the solar efficiency remains high and by far exceeds that of opaque perforated collectors, and even exceeds that of glazed collectors, sometimes for less than half the cost. That can be readily appreciated from FIG. 8. More particularly, it can be seen that for flow rate between 2 and 6 cfm per square foot of perforated surface, the efficiency of a perforated glazing with a black backing surface is greatly superior to that a conventional black perforated sheet metal solar collector. The difference in performance is even more noticeable for light or white color solar collectors. The perforated glazing with a white color backing surface is up to 100% more efficient than a white perforated sheet metal collector. It can also be appreciated that the difference in performance between conventional unglazed perforated collectors and the above described perforated glazed designs is even more significant at low flow rates of, for instance, 3 or 4 cfm per square foot.

As shown in FIGS. 9 a and 9 b, secondary outdoor air intakes 20 g, 20 g′ may be provided at strategic locations on a glazed solar air collector 10 g to avoid/minimize efficiency losses due to hot air stagnation and heat radiation from the solar absorber 14 g. Applicant has uncovered that under certain conditions, a non-negligible portion of the solar energy absorbed by the solar absorber 14 g may radiate back into the environment through the perforated glazed cover 12 g. Such heat losses may occur when the absorber 14 g heats up as a result of an unbalance or insufficient airflow over the surface thereof. Unbalanced or insufficient airflow liner velocity over the solar absorber 14 g results in hot spots, high radiation and thus low efficiency of the collector 10 g. The hot spots generally correspond to absorber areas where the air flow is too low to ensure heat transfer to the incoming air. The absorber should stay as cool as possible to minimize infrared heat losses to the outside. This problem may be overcome by increasing the linear speed of the flow of fresh outside air over the surface the solar absorber 14 g. This may be accomplished by providing outdoor air openings in the bottom wall of the collector, as depicted by arrows 20 g in FIGS. 9 a and 9 b. Outdoor air openings may also be provided in the lateral side edges of the collector 10 g, as depicted by arrows 20 g′ in FIG. 9 b. The lateral air openings may be disposed substantially in alignment with the outlet 18 g provided at the upper end of the plenum 16 g. The flow of outside air admitted through the secondary air openings is used as a motive flow to entrain the main flow of outside air admitted through the front glazed cover 12 g towards the outlet 18 and, thus, avoid air stagnation inside the plenum 16 g. The size, the number and the location of secondary outdoor air openings are selected to provide sufficient linear cooling air flow to remove heat from the solar absorber 14 g. The locations of the secondary outdoor air intakes may be determined by first establishing a temperature distribution profile (thermography) of the entire surface of the solar absorber, identifying hot spots/overheated areas. The temperature profile can be obtained from a computer model of the collector. Once the temperature distribution profile over the surface of the solar absorber is known, then the locations of the secondary outdoor air intakes may be selected to direct additional outside air to the areas of the solar absorber which are more prone to overheating. This provides for balance airflow over the entire surface of the solar absorber.

FIG. 10 illustrates another solution to efficiency losses due to hot air stagnation and heat radiation. This solution may be used alone or in combination with the solution illustrated in FIGS. 9 a and 9 b. As shown in FIG. 10, the solar absorber 14 h may be installed at an angle relative to the front perforated glazed cover 12 h to define a gradually widening plenum 16 h therebetween. According to the illustrated embodiment, the plenum 16 h has a flaring profile from bottom to top. The width of the plenum 16 h gradually increases towards outlet 18 h. The flaring profile of the plenum 16 h is designed to guarantee a minimum linear velocity of air over the entire surface of the solar absorber 14 h, thereby avoiding the formation of overheated areas thereon. A convection coefficient

h

can be calculated off the surface of the collector to ensure enough heat is removed from the solar absorber to reduce radiation losses. Applicant as found that a linear velocity of at least 2 m/s is efficient.

FIG. 11 illustrates a perforated glazing panel 30 that may be assembled with other similar panels to form the perforated glazed cover of anyone of the above described embodiments. For instance, similar panels may be mounted in a vertical coplanar relationship over a metal frame structure affixed to a building wall. The panel surface may have a frosty or sand-blasted like finish so that the underlying metal frame structure supporting the panel on the building wall is not visible through the panel. The perforated glazed panel 30 may be injection molded. The panel 30 may be molded from polycarbonate. All the features of the panel 30, including the perforations, are built-in in a single injection phase, thereby avoiding the need for subsequent manufacturing operations, such as drilling, milling, cutting or polishing. The perforated panels 30 are made with smallest hole diameter possible which could fit the polycarbonate injection process. For instance, a panel could be injection molded with perforations having a diameter as small as 2 mm (0.08 inches). The panel 30 comes out of injection ready to install.

As shown in FIGS. 12 a, 12 b, and 12 c, the panel 30 is molded with a male projection or tongue 32 projecting integrally from an upper edge of the panel and a corresponding female groove 34 extending along its lower edge. In this way, similar panels may be vertically assembled together in a tongue and groove fashion. An intermediate metal extrusion (not shown) may be fitted over the tongue 32 for engagement with groove 34 of the next upper panel. By providing the tongue 32 at the upper edge of the panel 30 and the groove 34 at the bottom edge thereof, water or ice infiltration between vertically adjacent panels may be avoided.

Referring now concurrently to FIGS. 11 and 13, it can be appreciated that expansion clips 36 a, 36 b may be integrally built with the panels to accommodate thermal expansion. The polycarbonate panels and the underlying metal support structure (typically aluminium) on the building wall have different thermal expansion coefficients. However, the glazed cover formed by the panels must remain flat and even at all time. Horizontal and vertical expansion clips are provided for accommodating horizontal and vertical expansion. A pair of vertical expansion clips 36 b may be integrally formed on the tongue 32 at the upper edge of the panel 30. First and second pairs of horizontal expansion clips 36 a are provided on opposed side edges of the panel 30. Each clip may be provided in the form of a resilient finger adapted to be compressed into an associated seat or recess 38 when under a predetermined load and to spring back to its original projecting position upon removal of the load. The vertical clips 36 b are adapted to be spring loaded against contraction of the metal framing during the night or winter, whereas the horizontal clips 36 a are adapted to be spring loaded against potential contraction of horizontal movements of the structure. This ensure the collector surface, which can often be facades of several hundred square meters, stay flat at all times of the day or season. The clips are meant to give in when there is contraction caused by the ambient cold air and released again when there is expansion back into normal position.

FIG. 14 is a cross-section taken into one hole 40 of the perforated glazed panel 30 and illustrating the airflow dynamics as outdoor air is being drawn through the perforated glazed panel. Each hole 40 has a raised inlet end 42 projecting outwardly from the front outdoor surface 46 of the panel 30 and an outlet end 44 which finishes flush with the inwardly facing surface 48 of the panel 30. The raised inlet end may be provided in the form of a semi-spherical protuberance or bulge extending around the periphery of each hole 40. A fillet 50 may be provided between each protuberance and the outdoor surface 46 to provide a smooth transition therebetween. The protuberances are integrally molded by increasing the thickness of the panel around each hole 40. As depicted by arrows 52, the protuberances promote turbulence as incoming outside air is drawn through the holes 40 into the plenum of the collector as depicted by arrows 54. In fact, the protuberances increase the turbulence radius zone around the holes. The temporary contraction of the air stream and the acceleration of the air flow as the air goes though the holes is called the Venturi effect. Turbulences occur around the holes on the top and especially at the back of the plate when the air expands after contraction in the holes. Since the desired goal is to remove more heat from the top and the back of the transparent glazed cover of the collector, the protuberances act as an additional means of promoting air turbulence.

As shown in FIGS. 15 and 16, the protuberances 42 also act as a rain and ice protection around the holes 40 to prevent ice and rain infiltration. As shown by arrows 56 in FIG. 15, the protuberances 42 cause rain water to flow around the holes 40, thereby leaving the inside of the collector substantially dry. Also during winter time, ice 58 forms around holes 40, leaving the holes unobstructed for air flow.

It will be apparent to one skilled in the art that modifications may be made to the illustrated embodiments without departing from the spirit and scope of the invention as hereinafter defined in the claims. 

1. A method of improving the efficiency of a glazed solar collector comprising a glazed cover, a solar absorber disposed behind the glazed cover, and a plenum between the glazed cover and the solar absorber, the glazed cover forming an outer surface of the collector; the method comprising: providing multiple perforations through the glazed cover; and reducing heat looses to the environment through the glazed cover by minimizing a temperature delta across the glazed cover, including cooling the glazed cover by drawing outside air through the multiple perforations at a flow rate between about 2 to about 6 cfm per square foot of glazed surface.
 2. The method of claim 1, further comprising cooling the solar absorber by providing a secondary flow of outside air over a front face of the solar absorber, including admitting outside air via at least one outdoor air intake provided at least one of a bottom and a side edge of the collector and, the air drawn through the perforations in the glazed cover and the air admitted through the outdoor air intake mixing together inside the plenum.
 3. The method of claim 2, wherein the at least one outdoor air intake comprises a set of lateral air intakes provided in opposed side edges of the collector, and a set of bottom air intakes provided in the bottom edge of the collector, and wherein providing a secondary flow of outside air comprises admitting outside air through the bottom and the side edges of the collector.
 4. The method of claim 1, further comprising increasing a linear velocity of the outside air flowing over the solar absorber by providing a secondary flow of outside air in the plane of the solar absorber, the secondary flow entraining the flow of outside air drawn through the perforations of the glazed cover.
 5. The method of claim 1, comprising balancing the flow of outside air over the solar absorber by providing additional outdoor air intakes in areas of the solar absorber prone to overheating.
 6. The method of claim 1, comprising establishing a temperature distribution profile over the surface of the solar absorber, identifying surface areas prone to overheating, and increasing air flow over said surface areas.
 7. The method of claim 1, comprising balancing the flow of outside air over the solar absorber by gradually increasing a width of the plenum towards an outlet thereof.
 8. The method of claim 1, comprising ensuring a minimum linear velocity of air over an entire surface area of the solar absorber by varying a distance between the glazed cover and the solar absorber along a length of the plenum, the distance being minimal at locations remote from an outlet of the plenum.
 9. The method of claim 1, comprising increasing turbulences over an outwardly facing surface of the glazed cover by providing protuberances around an inlet end of the perforations, the protuberances projecting outwardly from the outwardly facing surface of the glazed cover.
 10. A glazed solar air collector comprising a perforated glazed cover transparent to solar radiation, the perforated glazed cover having opposed front and back faces, the front face of the perforated glazed cover forming an external surface of the collector and being directly exposed to the ambient, a solar radiation absorbing panel disposed behind the perforated glazed cover for absorbing solar radiation passing through the perforated glazed cover; a plenum defined between the back face of the perforated glazed cover and an opposed front face of the solar radiation absorbing panel, the perforated glazed cover having a plurality of perforations distributed over a surface area thereof and collectively forming a main outdoor air intake for admitting fresh outdoor air into the plenum, the distribution of perforations being selected to maintain a temperature delta across the perforated glazed cover close to zero, a secondary outdoor air intake provided at least one of a bottom and a side of the collector and disposed to direct an additional flow of outdoor air over predetermined surface areas of the solar radiation absorbing panel prone to overheating, and air moving means to draw heated air from said plenum via an outlet thereof.
 11. The glazed solar air collector defined in claim 10, wherein the perforated glazed cover comprises a plurality of perforated glazed panels assembled in a coplanar relationship.
 12. The glazed solar air collector defined in claim 11, wherein each of said perforated glazed panels has top and bottom edges extending between opposed side edges, and wherein a tongue is provided along said top edge for engagement with a corresponding groove extending along the bottom edge of an adjacent panel.
 13. The glazed solar air collector defined in claim 11, wherein thermal expansion clips project integrally outwardly from a perimeter of the panels to accommodate thermal expansion in a plane of the perforated glazed cover.
 14. The glazed solar air collector defined in claim 13, wherein each of the thermal expansion clips comprises a resilient finger adapted to be deflected in a seat formed in the panel and to spring back to its original outwardly projecting position.
 15. The glazed solar air collector defined in claim 13, wherein the thermal expansion clips comprise first and second vertical expansion clips projecting from a top edge of the panel and first and second pairs of horizontal expansion clips projecting from opposed side edges of the panel.
 16. The glazed solar air collector defined in claim 11, wherein protuberances are formed around the perforations in the panels, the protuberances projecting outwardly from an outer surface of the panels.
 17. The glazed solar air collector defined in claim 11, wherein each panel is provided in the form of an injection molded polycarbonate panel with built-in perforations, expansion clips and top and bottom tongue and groove adjoining edges.
 18. The glazed solar air collector defined in claim 10, wherein the plenum is at least partly delimited by a building wall, the plenum being sealed from inside air contained within the building, the plenum being solely fed with outdoor air.
 19. The glazed solar air collector defined in claim 10, wherein the perforations and the air moving means provide for a flow rate between about 2 to about 6 cfm per square foot of the collector. 